A satellite receives a signal from a transmitter at one location and forwards the signal to a receiver at some other location. By “bouncing” signals off satellites, satellite systems can provide communications virtually anywhere. Satellite systems can also be comparatively inexpensive because very little land-based infrastructure, such as, for example, telephone lines and cellular towers, is needed to cover very large areas and/or very long distances. These advantages make satellite systems ideal for a wide variety of applications, including high speed data communications.
In an exemplary satellite system, user terminals communicate with a “gateway” through a satellite. In such an exemplary system, the terminals and the gateway are typically on Earth, while the satellite is in orbit. The gateway provides access for the terminals to outside networks. For instance, the gateway may include an Internet access point so that the terminals can access the World Wide Web, send and receive email, etc., through the satellite and the gateway.
The various communications links in such a satellite system include an “uplink,” “downlink,” “forward link,” and “return link.” An uplink includes the signals that the satellite receives from Earth. A downlink includes the signals that the satellite sends down to Earth. A forward link includes the signals going from the gateway to the terminals, while a return link includes the signals going from the terminals to the gateway. In which case, the uplink includes two sub-links, both a forward link component and a return link component. The sub-links in the uplink are occasionally referred to herein as a forward/uplink and a return/uplink. And, the downlink includes two sub-links, both a forward link component and a return link component. The sub-links in the downlink are occasionally referred to herein as a forward/downlink and a return/downlink.
In order for the satellite to send and receive signals without the signals interfering with one another, the satellite often separates the uplink and the downlink into separate frequency bands. For illustrative purposes, consider that a satellite may be allowed to use 1 GHz of bandwidth with a center frequency of 12 GHz. In which case, any number of filtering techniques can be used to separate the 1 GHz bandwidth into two separate bands. If the bands are 500 MHz each, one band may be centered around 11.75 GHz and the other centered around 12.25 GHz. Using two separate bands, the satellite does not simply “bounce” signals. Rather, the satellite receives signals in one of the two bands, translates the signal to the other band, and retransmits the signal in that other band. A number of known techniques exist and can be used to translate a signal from one frequency band to another. It is noted that the uplink and downlink bands are neither necessarily nor typically frequency contiguous. For example, the Ku band has a noncontiguous allocation of frequencies as between the uplink and downlink portions. More particularly, the Ku band downlink frequency allocation is 11.7 GHz to 12.2 GHz, while the Ku band uplink frequency allocation is 14.0 GHz to 14.5 GHz.
In addition to separating the uplink from the downlink, the forward link is often handled differently than the return link in a satellite. For instance, a gateway usually has both a larger, higher quality transmitter and receiver than do the terminals. In view of the disparity between the receiver and transmitter of the gateway, and the receiver and transmitter of the terminal, it is often the case that the return traffic from the terminals is likely to need a different amount of gain, or amplification, in the satellite than does the forward traffic. In which case, additional filtering can be used to separate the forward and return links. Once separated, different amounts of gain can be applied to the uplink components (e.g., more gain applied to return uplink signals than to forward uplink signals).
A pair of transponders can be used to operate, respectively, with the signals of the forward and return links individually. One of the transponders can be designed to receive the forward uplink, filter out other frequencies, apply a first amount of gain, and translate the received signal to the forward downlink. The other transponder can be designed to receive the return uplink, filter out other frequencies, apply a second amount of gain, and translate the signal to the return downlink. A conventional transponder is usually designed to operate with a particular frequency band. In order to operate with a different frequency band, a different transponder design may be needed.
The amount of data which the forward and reverse links are designed to carry may be different. These differences are typically the result of the intended uses of the communication system. The growth of the Internet, and in particular the growth of web browsing, is one of the factors that can determine the allocation of bandwidth between the forward and reverse links in a communication system. The forward link is often designed to handle a larger volume of data than the return link. For instance, when a user clicks on a link to a website, a small amount of data travels in the return link through the satellite and the gateway, out to the Internet, to a server machine where the website is stored. Then, a comparatively large amount of data travels in the forward direction from the Internet, through the gateway and satellite, back to the terminal in order to display the information requested from the website. The ratio of forward to reverse traffic is typically designed to be around 8 to 1, 10 to 1, or 15 to 1. For instance, a satellite may have 10 times more frequency bandwidth allocated to forward traffic than to return traffic in order to carry 10 times more data in the forward link. Using a pair of transponders, one transponder would operate with a frequency band 10 times larger than would the other transponder.
The actual ratio of forward to reverse traffic, however, may change over time as new network applications and new groups of users, with different usage behaviors, develop. For instance, most web traffic is currently server-based, with a large number of users accessing data located at a much smaller number of servers. These users tend to have high forward to reverse traffic ratios because they usually receive a great deal more data than they transmit. If peer-to-peer web traffic emerges as a viable alternative however, many users will serve data as well as consume it, pushing the forward to reverse ratio toward 1 to 1. Email traffic and video conferencing are also examples of network traffic that could bring the ratio of forward to return traffic closer to 1 to 1.
A satellite typically has a service life in excess of 15 years. Over a 15 year time span, the actual forward-to-reverse ratio of data traffic may change dramatically. Changing the forward-to-reverse bandwidth, or channel capacity, allocations to accommodate a change in actual forward-to-reverse data traffic would normally involve replacing transponders. But, replacing transponders in a satellite in orbit is likely to be prohibitively expensive if not impossible. Therefore, a need exists for satellite-based programmable signal filtering.